A roller bearing comprises an outer ring, an inner ring, a plurality of rollers arranged between the outer ring and the inner ring, and a retainer for holding the plurality of rollers in general. The retainer for holding the rollers includes various kinds such as a resin retainer, a pressed retainer, a ground retainer, and a welded retainer depending on its material and manufacturing method, and those are used based on purposes and characteristics. In addition, the retainer is an integrated type, that is, it comprises one annular part in general.
Since a roller bearing that supports a main shaft of a wind-power generator on which a blade for receiving wind is mounted needs to receive a high load, the roller bearing itself is large in size. Thus, since each member constituting the roller bearing such as a roller and a retainer is also large in size, it is difficult to produce and assemble such member. In this case, when each member can be split, it can be easily produced and assembled.
Here, a split type retainer that is split along a split line extending in an axial direction in a roller bearing is disclosed in European Patent Publication No. 1408248A2. FIG. 14 is a perspective view showing a retainer segment of the split type retainer disclosed in the European Patent Publication No. 1408248A2. Referring to FIG. 14, a retainer segment 101a has a plurality of pillar parts 103a, 103b, 103c, 103d and 103e extending in an axial direction so as to form a plurality of pockets 104 for holding rollers, and connection parts 102a and 102b extending in a circumferential direction so as to connect the plurality of pillar parts 103a to 103e. 
FIG. 15 is a sectional view showing a part of a roller bearing containing the retainer segment 101a shown in FIG. 14. The constitution of a roller bearing 111 containing the retainer segment 101a will be described with reference to FIGS. 14 and 15. The roller bearing 111 has an outer ring 112, an inner ring 113, a plurality of rollers 114, and the plurality of retainer segments 101a, 101b and 101c holding the plurality of rollers 114. The plurality of rollers 114 are held by the plurality of retainer segments 101a and the like in the vicinity of a PCD (Pitch Circle Diameter) 105 in which the roller movement is most stable. The retainer segment 101a for retaining the plurality of rollers 114 is arranged such that the pillar parts 103a and 103e provided at outermost positions in the circumferential direction abut on the circumferentially adjacent retainer segments 101b and 101c having the same configuration. The plurality of retainer segments 101a, 101b and 101c are continuously lined with each other and incorporated in the roller bearing 111, whereby one annular retainer is formed in the roller bearing 111.
The above one annular retainer is formed by lining the plurality of retainer segments continuously with each other in the circumferential direction. When the plurality of retainer segments is continuously lined with each other in the circumferential direction to form the one annular retainer, a circumferential gap is needed because of the thermal expansion and the like.
Here, when this circumferential gap is too large, the retainer segment moves largely in the circumferential direction and the adjacent retainer segments collide with each other, which could cause noise and the retainer segment to be damaged. Meanwhile, as the retainer segment thermally expands according to the temperature rise, when the circumferential gap is narrow, there is no gap between the adjacent retainer segments because of the thermal expansion, causing the adjacent retainer segments to press strongly each other to reach a deadlock. The circumferential stress due to the thermal expansion causes the friction and abrasion of the retainer segment, which also causes the retainer segment to be damaged.
Here, according to the European Patent Publication No. 1408248A2, when the retainer segments abut on to be continuously lined with each other in the circumferential direction, the circumferential gap dimension can be appropriate by setting the last gap dimension between the first retainer segment and the last retainer segment to not less than 0.15% but less than 1% of the circumference of a circle passing through the center of the retainer segment.
However, since each retainer segment is manufactured separately, each retainer segment has a dimensional deviation in the circumferential direction. When the retainer segments having such dimensional deviation are arranged so as to be continuously lined with each other in the circumferential direction, the dimensional deviation are accumulated. Thus, each retainer segment has to be manufactured with high accuracy to keep the circumferential gap dimension within the above predetermined range, which causes the productivity of the retainer segment to deteriorate and accordingly causes the productivity of the roller bearing to deteriorate.
In addition, according to the European Patent Publication No. 1408248A2, the two adjacent retainer segments are continuously lined such that the pillar parts provided at outermost positions in the circumferential direction abut on each other. Thus, the two pillar parts are in series with each other between the two adjacent retainer segments. In this case, since the circumferential space in which the retainer segment is arranged is limited, it is necessary to reduce the number of rollers in the roller bearing accordingly. Therefore, the roller bearing cannot contain many rollers and cannot receive a high load.